Processes exist for automatic location or addressing by reading conventional magnetic bands or tapes of the type used in tape recorders or by counting electro-luminescent marks as for the sorting of letters. Such apparatus requires a precise position of the support with respect to the marking apparatus and generally a constant speed of displacement. They are, therefore, poorly adapted to the reading of labels carried by articles of very varied shape and orientation, such as packages or sacks.
Also known is the capability of detecting on an object the presence of a sample, even of very small size, of a metal having particular magnetic properties and without direct contact of the object with the detection apparatus.
Thus, French Pat. Nos. 763,681 and 2,055,019 disclose processes and means especially adapted to detect the theft of books from public libraries. According to these patents, there is concealed in the object to be protected a magnetic metallic band of an alloy of "permalloy" type and the users of the library must, in order to leave, pass through the interior or in the vicinity of a framework forming a detector of the presence of the concealed magnetic band. For this purpose, the framework carries an excitation coil producing an alternating magnetic field and a balancing detecting coil such that normally no signal appears. The presence in the framework of a book carrying the magnetic band causes the appearance of an unbalancing magnetic induction which unbalances the detecting coil and produces a signal that can be detected by conventional means.
Unfortunately, such apparatus, conceived primarily for anti-theft, only permits the detection in the control zone of the simple presence of a marked object. The problem posed by the automatic sorting of mail sacks or parcels is primarily to be able to identify each of the objects for subsequent distribution to different zones corresponding, for example, to the usual routing directions. Currently, the different parcels or sacks are provided with a label with an identification of the destination and the sorting is effected, most often, by an agent in front of whom the different sacks are passed and who by visual reading of the labels manipulates different levers for distributing the objects in the desired directions.